1. Field of the Invention
The present invention relates to a conductive paste used for forming a desired conductive pattern on a substrate, and to a circuit substrate formed by use of the paste.
2. Background Art
Recently, strongly demanded are high-frequency electronic components used in mobile communication devices, satellite broadcasting receiving devices, computers having small size and high performance. Concurrently, wiring patterns of high-frequency electronic components are required to have increased density and signal speed. In order to increase density and signal speed in the wiring pattern, the pattern must be made finer and yet form a thicker film.
A conventional method for forming a wiring pattern of a high-frequency electronic component is firstly a wiring pattern is formed on an insulating substrate by use of a conductive paste comprising a conductive metallic powder of a multivalent metal such as iron or copper and an organic vehicle containing an organic binder and an organic solvent. Subsequently, the resultant pattern is dried and fired. In the method, a wiring pattern is generally formed through screen printing, and the lower limit of the width and pitch of the thus-formed wiring pattern is approximately 50 .mu.m.
Japanese Patent Application Laid-Open (kokai) Nos. 287221/1993 and 227153/1996 disclose a method for forming a fine thick-film wiring pattern through photolithography by use of a photosensitive conductive paste. According to the method, a photosensitive conductive paste is applied onto an insulating substrate and the paste is dried and subjected to patterning by means of photolithography. The paste contains a conductive metallic powder, an acrylic copolymer having one or more carboxyl groups and one or more ethylenic unsaturated groups in the side chains, a photoreactive compound and a photopolymerization initiator.
Meanwhile, Japanese Patent Application Laid-Open (kokai) Nos. 224538/1994 and 335757/1996 disclose a method for forming a fine thick-film wiring pattern through photolithography by use of a photosensitive conductive paste containing a glass powder. According to the method, adhesion between the conductive pattern and a ceramic substrate is enhanced by incorporating a glass powder into a photosensitive conductive paste.
Recently, in consideration of the environment, it has been demanded that development be carried out by use of water or an aqueous alkali solution in photolithography making use of a photosensitive conductive paste. Therefore, the organic binder contains an acidic functional group such as a carboxyl group, which group has a property to release a proton.
However, in the case of employment of such an organic binder, ions of a multivalent metal, which ions are released into solution, may react with anions of the organic binder, which anions are formed after protons are released, and a three-dimensional network structure may be formed by ionic cross-linking to thereby form a gel. When the photosensitive conductive paste becomes a gel, application of the paste becomes difficult. Even if application of the paste can be carried out, development of the paste may be unstable, making employment of the paste difficult. In order to prevent gelation of a photosensitive conductive paste, Japanese Patent Application Laid-Open (kokai) No. 218509/1997 discloses a method in which a phosphorous-containing compound such as phosphoric acid, serving as a gelation-suppressing agent, is incorporated into the paste; Japanese Patent Application Laid-Open (kokai) No. 218508/1997 discloses a method in which a compound having an azole structure such as benzotriazole, serving as a gelation-suppressing agent, is incorporated into the paste; and Japanese Patent Application Laid-Open (kokai) No. 222723/1997 discloses a method in which an organic compound having a carboxyl group, such as acetic acid, serving as a gelation-suppressing agent, is incorporated in the paste. However, these methods enable only slight retardation of gelation of the paste and even when the paste contains the gelation-suppressing agent, use of the paste is difficult in practice.
In addition, Japanese Patent Application Laid-Open (kokai) No. 171107/1998 discloses a method in which 3-methyl-3-methoxybutanol serving as an organic solvent is incorporated into the paste in order to prevent gelation of the photosensitive paste. However, 3-methyl-3-methoxybutanol has a low boiling point of 174.degree. C. and thus, when the paste is dried after application, the organic solvent component completely vaporizes and the effect of preventing gelation may not be exhibited. Thus, a phenomenon similar to gelation may occur in the dried paste, i.e., a three-dimensional network structure may be formed by ionic cross-linking, and the molecular weight of the paste may become substantially high. As a result, problems may occur. For example, unexposed portions of the paste may fail to dissolve into a developer.